New Features
* more type annotations
* `ep.Tensor` now points to the base class
* `ep.TensorType` now points a `TypeVar` bounded by `ep.Tensor`
* added `astensors`, `astensor_` and `astensors_`
* the underscored variants return a `restore_type` function that inverts the `astensor` operation
* more tests
* stricter mypy config